Active Protection Systems (APS) are small air defense systems that provide protection for ground vehicles such as tanks against anti-tank guided missiles (ATGMs) and rocket propelled grenades (RPGs). Therefore, APS present a potential threat to the effective lethality of anti-armor missiles. To counteract the APS, missile-borne counter-active protection system (CAPS) jammers have been developed. The aim of such jammers is to disable the APS fire control radar so that the targeted ground vehicle remains vulnerable to the missile attack. A representative architecture of a CAPS jammer is depicted in FIG. 1. In a typical operation mode, any emissions from APS enemy radar are monitored by receiving antenna 101 in conjunction with activity detector 103. When such emissions are intercepted, indicating the presence of an enemy radar, then jamming signal is generated by jammer techniques generator 107 and amplified to a high level in transmitter amplifier 109 and subsequently radiated toward the enemy radar to disable the enemy radar. The functions of the generator and amplifier are controlled by jammer controller 105. The jamming signal is intended to interfere with the enemy radar's reception of its own signal being reflected from the surface of the incoming missile. With successful interference, the enemy radar is rendered incapable of accurately tracking the missile and initiating the destruction of the missile before it can strike the target vehicle.
A counter-active protection system jammer has been developed that utilizes conformal end-fire (surface wave) antennas on the exterior of the missile and the electronics already within the missile. With this jammer on board, the seeker missile homes on the selected target while simultaneously jamming the enemy radar. The jamming process, however, presents several problems when the seeker employed by the missile is an active radio-frequency (RF) seeker. The problems are particularly acute when the frequencies of the jamming signals are very close to the operating band of the RF seeker itself.
One problem is that the conformal antennas do not function well on a missile that has a transmissive radome in front of the seeker to protect the seeker hardware. Normally, in the absence of a radome, the conformal antennas used in the CAPS create radio frequency currents on the missile's conductive skin that propagate forward. These propagated currents eventually are launched into the air as radiated emissions at the front of the missile. With an electrically conductive front-end of the missile (i.e. without the transmissive radome), the radiated energy is present even on the side opposite from where the antennas are located since some of the RF current does not become launched until it has traveled around the nose of the missile to the other side. In a sense, the surface wave covering both sides of the missile makes the entire front-end of the missile an antenna.
However, when a transmissive radome covers the seeker at the front-end of missile 201, the radome, not being electrically conductive, does not support the wave propagation to the other side of the missile. What results is antenna pattern shadow on that side. Antenna pattern shadowing is a significant problem because the angle between the missile body and the line of sight to the target may be up to +/−20 degrees due to the flight trajectory of the missile. This effect is illustrated in FIGS. 2 and 3. This means that if the required jamming spatial angle is greater than the coverage angle of the transmitting antenna, the likelihood of successful jamming of the enemy radar is very low, with a corresponding increase in the likelihood of successful tracking of the missile by the enemy radar.
Another significant problem occurs when the jamming signals interfere with the seeker signals. Most RF seekers on missiles employ pulse Doppler radars. In a pulse Doppler radar, a transmit pulse is radiated followed by an interval for receiving. The receiving (or listening) interval is followed by another transmit pulse. This sequence is repeated. Therefore, the seeker is never transmitting and receiving simultaneously. The interference between the jammer and the seeker can occur when the transmit pulses from the seeker (or skin reflections of the transmit pulses) are confused with emission from the enemy radar, or when the jammer energy jams the seeker. Such confusion and unintentional jamming can occur if the enemy radar, jammer and the missile seeker all emit signals in the same or close frequency band. The false signals may set off the detector and jammer in the missile to no useful purpose. Further, the transmit pulse from the seeker can cause power-handling difficulties for the jammer since they have the potential to overdrive the activity detector.
Yet a third problem occurs when the jammer energy enters the seeker receiver through the seeker antenna backlobes and raises the interference level to such that the seeker receiver's minimum detectable signal required for function is increased. Such desensitization of the seeker receiver greatly decreases the seeker effectiveness with corresponding reduction in the target detection range and tracking accuracy.